CSS：
<style type="text/css">
.box{
position:relative;
width:100%;
height:500px;
background:#ccc;
}
.content{
position:absolute;
top:50%;
left:50%;
/*margin-top:-100px;
margin-left:-100px;*/
transform:translate(-50%,-50%);
height:200px;
width:200px;
background:red;
}
</style>
HTML：
<body>
<div class='box'>
<div class="content"></div>
</div>
</body>